1. Field of the Invention
The present invention relates to injection control for an injection molding machine, and, more particularly, to control of switching from the filling process to the packing process.
In these days, a molten resin or the like is injection-molded as shown in FIG. 4 which is a graph, the axis of the abscissa referring to position l or time t and the axis of ordinate referring to the forward movement velocity V cf the injection screw and pressure P. As shown in FIG. 4, the injection molding process is composed of a filling process L1 in which a mold cavity is filled with a molten resin at high speed and a packing process L2 in which pressure is applied after the mold cavity has been filled with the molten resin so as to form a desired shape. Generally, in the filling process L1, the pressure regulating valve of the hydraulic pressure circuit is fully opened to a large degree, and setting is so made that the degree of opening of a flow rate control valve is changed with predetermined position and time. This is done so as to change the velocity to one or more steps while making the lapse of predetermined position and time from the moment at which the injection has been started or the forward movement position of the injection screw or the like as the reference. Furthermore, the flow rate control valve is adjusted so as to control the piston of the injection cylinder and thereby control the forward movement velocity of the injection screw or the like. As a result, the cavity is filled with the molten resin at a high velocity at which the resin in the cavity does not contain air. The aforesaid packing process L2 is arranged in such a way that the state in which the flow rate control valve is opened in a condition whereby the operation of the pressure regulating valve is operable is fixed after the cavity has been filled with the molten resin and the hydraulic pressure is adjusted by the pressure regulating valve. In this packing process L2, the degree of opening of the pressure regulating valve is changed so that a predetermined pressure is applied to the molten resin which has filled the cavity. Then, the molten resin is cooled in the cavity, causing the resin to shrink. Therefore, an error that has taken place in the shape and dimensions of the product with respect to the dimensions of the cavity can be prevented and also excessive generation can be presented.
When the filling process is switched to the process, a delay of the switching timing will cause burrs on the product to be formed. On the contrary if the switching is performed prematurely, shrinkage or shortage takes place. Therefore, the switching operation must be performed at proper timing. Accordingly, a control method has been employed in which, as shown in FIG. 5, which the axis of abscissa stands for time t and the axis of ordinate stands for pressure P; the filling pressure realized by the hydraulic circuit in the filling process and in the packing process are detected; a rise in the hydraulic pressure which has taken place due to the completion of filling of the molten resin in the filling process is detected; and switching is performed when the hydraulic pressure has been raised to predetermined level P0.
A method has thus been employed in order to prevent erroneous switching to the packing control performed when the hydraulic pressure has been raised to the predetermined level P0 due to a temporary rise in the filling pressure caused by a disturbance in a state where filling of the cavity with the molten resin has not been complete. In this method, switching is performed when the hydraulic pressure has been raised to the predetermined level P0 after a predetermined time has passed from the commencement of the injection. Also, a zone control has been employed in which switching is performed when the hydraulic pressure is raised to the predetermined level P0 by detecting the filling pressure after the injection screw has moved forward to a predetermined position. Furthermore, there has been a control method in which the pressure of the injection of molten resin, the hydraulic pressure in the injection hydraulic circuit or the degree of opening between the fixed mold and the movable mold plate is detected so as to perform an adjustment operation in such a manner that the pressure in the packing process is corrected if the detected level exceeds a predetermined range (see, for example, Japanese Patent Publication No. 53-39905).
As described above, in the conventional control method in which the control is switched from the velocity control to the pressure control according to the filling pressure, switching is controlled by detecting a fact that the filling pressure or the like is raised to a predetermined level, and the hydraulic pressure is adjusted at the next injection molding operation if the pressure rises to an abnormal level. However, the conventional injection control has been too unsatisfactory to mass-produce precise products free from distortion by a reduction in the internal stress.
In a case where the zone control is employed, expert knowledge is required to set the zone range and the predetermined level P0 of the pressure, and what is worse, the preparation operations (such as, the initialization for the injection operation, become too complicated and too difficult because the predetermined level P0 becomes different according to the shape and the size of the mold cavity and the type of the resin.